Heat Spreader with Thermal Conductivity Inversely Proportional to Increasing Heat

ABSTRACT

A heat spreading apparatus includes a body defining a void. A fluid is positioned within the void for distributing heat by vaporizing the fluid. The body defines a void with a heat accumulation surface geometry to disrupt the thermodynamic cycle of vaporizing the fluid and thereby diminish heat spreading activity by the heat spreading apparatus.

FIELD OF THE INVENTION

This invention relates generally to a heat distribution device used in connection with a heat generating surface. More particularly, this invention relates to a heat spreader that has a thermal conductivity that is inversely proportional to increasing heat applied to it.

BACKGROUND OF THE INVENTION

U.S. Pat. Nos. 6,167,948 and 6,158,502 disclose thin, planar heat spreaders in various configurations. These heat spreaders endeavor to have improved thermal conductivity with increased exposure to heat. In some engineering applications it is desirable to have decreased thermal conductivity with increased exposure to heat. Accordingly, it would be desirable to provide a heat spreader that achieves this counterintuitive result.

SUMMARY OF THE INVENTION

A heat spreading apparatus includes a body defining a void. A fluid is positioned within the void for distributing heat by vaporizing the fluid. The body defines a void with a heat accumulation surface geometry to disrupt the thermodynamic cycle of vaporizing the fluid and thereby diminish heat spreading activity by the heat spreading apparatus.

BRIEF DESCRIPTION OF THE FIGURES

The invention is more fully appreciated in connection with the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a section view of a planar heat spreader.

FIG. 2 illustrates a segment of the heat spreader of FIG. 1 with incipient bubble formation

FIG. 3 illustrates a segment of the heat spreader of FIG. 1 with increased bubble formation.

FIG. 4 illustrates a segment of the heat spreader of FIG. 1 with further increased bubble formation.

FIG. 5 illustrates a segment of the heat spreader of FIG. 1 with indentations to promote bubble formation.

FIG. 6 illustrates a segment of the heat spreader of FIG. 1 with hydrophilic properties to promote formation of a bubble with a first characteristic.

FIG. 7 illustrates a segment of the heat spreader of FIG. 1 with hydrophobic properties to promote formation of a bubble with a second characteristic.

FIG. 8 illustrates a heat spreader in accordance with an embodiment of the invention.

FIG. 9 illustrates a heat spreader in accordance with another embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a section view of a planar heat spreader 100 configured in accordance with an embodiment of the invention. The planar heat spreader 100 includes a first body 102 and a second body 104 which define a void 106. At least one surface of one body has a heat accumulation surface geometry 108.

The heat accumulation surface geometry disrupts the thermodynamic cycle of vaporizing fluid. Consequently, heat spreading activity by the heat spreader 100 is diminished with increasing heat. The heat accumulation surface geometry may be in the form of indentations to promote bubble growth or surface treatments, such as hydrophilic surface treatments and hydrophobic surface treatments. The heat accumulation surface geometry may also be in the form of capillary wick structures, such as screens, sintered metals, grooves, arteries, planar capillaries and combinations thereof.

FIG. 2 illustrates second body 104 of the heat spreader 100 of FIG. 1. Fluid 202 has fluid flow paths 204 adjacent to a heat accumulation surface geometry to promote the formation of an incipient bubble 206. An embodiment of the invention promotes the formation of such bubbles to disrupt efficient thermal performance of the planar heat spreader 100.

FIG. 3 illustrates increased bubble formation. In particular, bubble 206 of FIG. 3 is larger than bubble 206 of FIG. 2. The bubble 206 grows larger with increased exposure to heat from the heat generating surface. This bubble of increased size dislocates fluid path 204, as shown with fluid dislocation segment 208.

FIG. 4 illustrates further increased bubble formation. In particular, bubble 206 of FIG. 4 is even larger than bubble 206 of FIG. 3. The bubble 206 grows larger with increased exposure to heat from the heat generating surface. This bubble of further increased size further dislocates fluid path 204, as shown with fluid dislocation segment 208.

Bubble 206 effectively has a liquid perimeter and a vapor interior. As shown in FIG. 4, the bubble 206 displaces fluid 202 from much of the surface of body 104. Consequently, the fluid 202 is exposed to less surface area of body 104, which is attached to a heat generating surface. The reduced surface exposure reduces vaporization and its concomitant heat transfer action. Thus, with increased temperature and increased bubble formation, the thermal conductivity of the device 100 is reduced. This stands in contrast to typical designs that endeavor to increase thermal conductivity in the presence of increased exposure to heat.

FIG. 5 illustrates body 104 of the heat spreader 100 of FIG. 1. Fluid 502 is adjacent to a heat accumulation surface geometry in the shape of indentations 504 to promote bubble growth. FIG. 5 illustrates a bubble 506 formed in one such indentation. Although the indentations are shown as spherical, they may be any shape, such as cylindrical, conical, or trapezoidal.

FIG. 6 illustrates body 104 of the heat spreader 100 of FIG. 1. Fluid 602 is adjacent to a heat accumulation surface geometry with hydrophilic properties to promote bubble formation. For example, a hydrophilic material, a hydrophilic film or hydrophilic surface features may be used to promote hydrophilic properties. A hydrophilic surface minimizes surface exposure to a liquid. Thus, bubble 604 forms with a relatively small footprint 606 on the surface of segment 104.

FIG. 7 illustrates body 104 of the heat spreader 100 of FIG. 1. Fluid 702 is adjacent to a heat accumulation surface geometry with hydrophobic properties to promote bubble formation. For example, a hydrophobic material, a hydrophobic film or hydrophobic surface features may be used to promote hydrophobic properties. A hydrophobic surface maximizes surface exposure to a liquid. Thus, bubble 704 forms with a relatively large footprint 706 on the surface 104.

The selection of a hydrophilic surface or hydrophobic surface is contingent upon the application and the desired configuration of the bubble. A single surface may include both hydrophilic and hydrophobic regions.

The foregoing examples illustrate the formation of a single or few bubbles. Alternate embodiments of the invention facilitate the formation of increased number of bubbles with increased exposure to heat.

Table I illustrates performance results achieved in accordance with an embodiment of the invention.

TABLE I Thermal Power Temperature Conductivity (W) (° C.) (W/m * K) 0.0 60.0 2410.5 9.9 60.9 2025.3 15.3 61.5 1820.1 25.0 62.8 1532.9 50.0 67.5 1064.9 75.0 75.0 760.0 100.4 85.6 577.1 125.0 99.4 323.0

Observe that this embodiment experiences thermal conductivity changes from 2410 to 323 (over a 7.5 thermal conductivity change) over approximately 40° C. (from 99.4° C. to 60° C.). Thus, unlike typical devices, thermal conductivity decreases with increasing heat exposure.

The techniques of the invention may be used to form heat transfer devices of various configurations. FIG. 8 illustrates a section view of one such device 800. Device 800 includes a first body 804 a second body 806 and vertical sidewalls 808 and 812 which define a void 802 for vapor flow. At least a portion of the bodies and sidewalls interior surfaces have a heat accumulation surface geometry 814. A fluid (not shown) is positioned adjacent to the heat accumulation surface geometry and vertical support 810. The heat accumulation surface geometry is configured for bubble formation, as previously described.

The sidewalls 808, 812 and vertical support 810 facilitate efficient heat transfer. This efficient heat transfer is countered by the heat accumulation surface geometry, which has a thermal conductivity that is inversely proportional to increasing applied heat.

FIG. 9 illustrates a section view of device 900 generally corresponding to device 800 of FIG. 8, but with additional thermal resistance promoting features. The device 900 includes a first body 904 a second body 906 and vertical sidewalls 908 and 912, which define a void 902 for vapor flow. At least a portion of the bodies and sidewalls interior surfaces have a heat accumulation surface geometry 914. A fluid (not shown) is positioned adjacent to the heat accumulation surface geometry and vertical support 910. The heat accumulation surface geometry is configured for bubble formation, as previously described.

The sidewalls 908, 912 and vertical support 910 have corresponding cut-outs 916, 918, 920, 922, 924, and 926 to reduce heat transfer efficiency. Specifically, these cut-outs reduce the heat flow cross-sectional area, and increase the heat flow length, reducing the heat transfer efficiency, which supplements the heat accumulation surface geometry design goal of thermal conductivity that is inversely proportional to increasing applied heat.

Embodiments of the invention rely upon a heat accumulation surface geometry that promotes dry out. Dry out is the absence of a fluid. The absence of a fluid in the heat spreading apparatus disrupts the thermodynamic cycle and thereby diminishes heat spreading activity. For example, dry out occurs when the fluid pressure from the condenser region is insufficient to provide enough fluid to the evaporator region. This leads to dry out in the evaporator. Dry out prevents the thermodynamic cycle from continuing and therefore heat spreading activity is diminished, thus satisfying the heat accumulation surface geometry design goal of thermal conductivity that is inversely proportional to increasing applied heat.

Techniques of the invention may be realized in a variety of configurations. For example, various capillary configurations are disclosed in the previously referenced U.S. Pat. Nos. 6,167,948 and 6,158,502, which are incorporated herein by reference.

The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed; obviously, many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, they thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention. 

1. A heat spreading apparatus, comprising: a body defining a void; and a fluid positioned within the void for distributing heat by vaporizing the fluid; wherein the body defines a void with a heat accumulation surface geometry to disrupt the thermodynamic cycle of vaporizing the fluid and thereby diminish heat spreading activity by the heat spreading apparatus.
 2. The heat spreading apparatus of claim 1 wherein the heat accumulation surface geometry promotes bubble growth.
 3. The heat spreading apparatus of claim 1 wherein the heat accumulation surface geometry promotes dry out.
 4. The heat spreading apparatus of claim 1 wherein the heat accumulation surface geometry has indentations to promote bubble growth.
 5. The heat spreading apparatus of claim 1 wherein the heat accumulation surface geometry includes a capillary wick structure.
 6. The heat spreading apparatus of claim 1 wherein the heat accumulation surface geometry has a hydrophilic surface.
 7. The heat spreading apparatus of claim 1 wherein the heat accumulation surface geometry has a hydrophobic surface.
 8. The heat spreading apparatus of claim 1 wherein the body has additional thermal resistance promoting features. 